1. Field of the Invention
The present invention relates generally to a transmissive-type display device ("transmissive display device"). More particularly, the present invention relates to a dot matrix type display device having a display panel and multiple picture elements ("pixels") arranged in a matrix to form a liquid crystal display ("LCD"), wherein the display panel is provided with an array of microlenses.
2. Description of the Related Art
In general, LCDs are comprised of upper and lower substrates facing each other as shown in FIG. 1. The lower substrate includes a plurality of pixel electrodes 13 formed on a transparent glass substrate 10. Data bus lines 12 are formed parallel to each other in a horizontal direction, and gate bus lines 11 are formed parallel to each other in a vertical direction. Between the data bus lines 12 and the gate bus lines 11, an array of the pixel electrodes 13 are formed.
On the transparent substrate 10, switching elements such as thin film transistors 15 ("TFTs") are disposed for the respective pixels at each crossing area where the gate bus lines 11 and the data bus lines 12 cross each other. The pixel electrodes 13 are electrically connected to the output electrodes (e.g., drains) of the TFTs 15.
On the other hand, the upper substrate includes a color filter layer 21 formed on a transparent glass substrate 20 and common electrodes 22 formed on the color filter layer 21. As shown in FIG. 2, the color filter layer 21 includes a red color filter 21R, a green color filter 21G, and a blue color filter 21B successively formed on the substrate 20. Among the different arrangements of color filters, the mosaic-array is employed in an audio video (AV) mode and the striped array is used in an office automation (OA) mode.
Once the upper and lower substrates are individually formed, it is necessary to join them for injecting liquid crystal 24 therebetween. The upper substrate and the lower substrate may be joined so that the color filter layer 21 faces the pixel electrodes 13 formed on the transparent glass substrate 10.
Additionally, a black matrix 14 is formed over the gate bus lines 11 and the data bus lines 12 corresponding to the border of the each color filter 21R, 21G and 21B. The black matrix 14 shields light which may have leaked from the gaps formed between the bus lines and the pixel electrodes 13, and improves the contrast of the LCD by making the borders of the color filters more clear.
Generally, the size of the black matrix 14 is larger than that of each bus line because of the misalignment arising from joining the upper substrate with the lower substrate. The gate bus lines 11 and the data bus lines 12 are approximately 15 .mu.m-40 .mu.m and 10 .mu.m-25 .mu.m wide, respectively. Therefore, the black matrix 14 is slightly wider than the bus lines.
In the conventional LCDs having the above described elements, a light source is located at the backside of the transparent glass substrate 20. The black matrix 14 is formed on the transparent glass substrate 10 to cover the gate bus lines 11 and data bus lines 12. The light from the light source, as depicted with a straight line in FIG. 2, is transmitted through the transparent glass substrate 20, the color filters 21R, 21G and 21B, the common electrodes 22 and the liquid crystal 24, sequentially. This light passes through the portion of transparent glass substrate 10 having the pixel electrodes 13 thereon. But, the light impinging on the gate and data bus lines 11 and 12 are blocked by the black matrix 14. As a result, the aperture ratio of the LCD and the brightness of the device is reduced.
The aperture ratio is expressed by "the effective area of all the pixels" divided by "the total display area". The aperture ratio equals the ratio of the recoverable light to all incident light (recoverable and unrecoverable light). (The unrecoverable light is the light blocked by the untransmissive portion of the display panel, and does not contribute to displaying.) As the size of the untransmissive portion increases, the aperture ratio decreases. The reduced aperture ratio leads to reproduction of dark pictures and poor image quality.
The LCDs may include a storage capacitor for assisting the cell capacitance of the LCDs. There are two types of storage capacitors. One is a storage-on-common type in which the storage capacitor is formed separately. The other is a storage-on-gate type in which a portion of gate line functions as a storage capacitor electrode. The former has a smaller effective area for forming the pixels than the latter. Therefore, the aperture ratio of such LCDs and the brightness of the display device is reduced.
In order to refine pictures on the display, the brightness of the backlight must be increased and the size of the untransmissive portion must be minimized. To increase the brightness of the backlight, more electricity (power) is required; however, such is undesirable because it is costly.
Many different methods have been developed to improve the aperture ratio of the LCDs, e.g., enlarging the area of pixel electrodes or enlarging the pixel size. To enlarge the pixel size, however, the other elements of the LCD such as gate bus lines, source bus lines, TFTs and so on, need to be minimized. But, photo-lithography and etching has a limit on minimizing these elements. Further, the width of bus lines cannot be reduced below a certain level. Therefore, it is difficult to manufacture LCDs with an improved aperture ratio. But, even if the pixel size were increased by the above methods, the aperture ratio is generally 40% or 50% at best.
To solve the problems described above, an LCD with a different structure has been proposed in which the display panel with an array of microlenses are formed on one side or both sides of the panel. Such a structure is disclosed in Japanese Laid-Open Patent Publications No. 60-262131 and No. 61-11788. Referring to FIG. 3, one of the advantages of such known display devices is that the light rays incident onto the portion of display panel which does not contribute to displaying, are focused on the pixel electrodes using elements 31 and pass through elements 32. As a result, the transmittance of the LCD having the same aperture ratio is increased.
Another proposal for further enhancing the above mentioned device is disclosed in U.S. Pat. No. 5,187,599. Referring to FIG. 4, such a display device comprises a first array of microlenses 31' disposed on the incident side of the display panel, and a second array of microlenses 32' disposed on the incident side of the other display panel, each microlens being disposed according to the respective pixels. The focal points of the first array of microlenses are identical with those of the second array of microlenses, and the focal length of each microlens in the first array is greater than that of the second array. Therefore, the light rays incident on the untransmissive portion of the display panel is redirected by condensing the diverging rays.
The above suggested structure of the LCD are. to increase the transmittance of the light and to acquire the effect of having an increased aperture ratio, without actually increasing the aperture ratio. Each microlens covers the entire pixel electrode. The height of the microlenses need to be greater than 50 .mu.m to cover the dimension of each pixel electrode having generally 100 .mu.m.times.300 .mu.m. However, in practice, it is difficult to form the LCD having microlenses greater than 50 .mu.m in height, resulting relatively flat lenses. Accordingly, the transmittance of conventional LCDs cannot be effectively improved.